File:Analysis of development (1955) (18166987592).jpg

Title: Analysis of development
Identifier: analysisofdevelo00will (find matches)
Year: 1955 (1950s)
Authors: Willier, Benjamin H. (Benjamin Harrison), b. 1890
Subjects: Embryology; Embryology
Publisher: Philadelphia, Saunders
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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Energy Exchange and Enzyme Development During Embryogenesis 525 during a substantial part of embryonic life, may be seen in a number of other cases. This was clearly shown by Gray ('26) for the ti'out embryo, and more recently Hayes, Wilmot, and Livingstone ('51) reported that the respiratory rate of the Atlantic salmon embryo remains unchanged throughout the entire period during which weight data could be obtained. The actively developing grasshopper embryo, which at certain stages can be completely separated from yolk, also has a constant respiratory rate over a con- siderable period of development (Bodine and Boell, '36a, '37). Of course, during diapause respiratory rate falls, but this special situa- tion will be discussed later. In the rat em- bryo, the Q02 is approximately 30 during the cleavage stages, but it soon falls to a value of about 12 (Boell and Nicholas, '48) which is maintained until around the fif- teenth day of development (Dickens and Greville, '33a; Negelein, '25). Respiratory Increase and Growth. It seems in those cases in which the yolk content of the embryo is relatively small, or where the yolky parts of the egg can be successfully separated from the embryonic materials, the respiratory rate of the embryo is uniform throughout a major part of the total period of development. In other words, as pointed out by Gray ('27), the total respiratory ex- change at a given stage of development is proportional to the amount of metabolicaliy active embryonic material. It is, therefore, perhaps more than mere coincidence that respiratory data during development should follow a sigmoid curve (Gray, '29a, b; Brody, '45; Thompson, '42). During its early phases, growth appears to be an exponential process—that is, ap- proximately the same percentage increase occurs during successive equal intervals of time. The equation x = a.e^'^ has been found empirically to fit growth data in a great many cases, and a plot of the logarithm of the magnitude of the growing entity against time thus yields a straight line.* When * It is not intended to attach any strict biological significance to the values of a or A: in the equa- tion X = a.e^'. The semilogarithmic plots of growth and respiratory data are intended as purely descrip- tive; the chief justification for their use lies in the fact that a convenient method is thereby provided for comparing and contrasting curves. In arithmetic plots, the similarities and differences between such curves are not always readily apparent. The reader is referred to Shell's ('54) paper in which is con- tained a thoughtful analysis of the utility and lim- itations of empirical curve fitting (see also Levy, '52). growth rate changes abruptly during devel- opment, a series of intersecting straight lines will result. If respiratory increase parallels growth of metabolicaliy active embryonic mass, and the evidence reviewed above strongly suggests such a relationship, the data shown in Figure 197 should yield a linear curve when log respiration is plotted against time (Fig. 200). The points do not
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Days Fig. 200. Semilogarithmic plot of oxygen con- sumption dunng development of the chick (data from Table 1 of Romanoff, '41). fall on a single straight line, however, for between the seventh and eighth days of de- velopment an inflection appears, and from then on respiratory increase proceeds at a lower rate than earlier. A similar inflection in the semilogarithmic plots of dry weight or wet weight also occurs on the eighth day. What is responsible for the inflection in the respiratory curve is not known. It is interesting to point out that it occurs at about the time that the embryo shifts from predominantly carbohydrate catabolism to protein (see p. 533), and that the rate of protein absorption for growth is minimal on the eighth day (Needham, '31, Table 111, column 13). Novikoff and Potter ('48) have obtained data on increase in PNA and DNA in the chick embryo, and similar results have been described by Reddy, Lombardo, and Cerecedo ('52). A semilogarithmic plot of Novikoff and Potter's PNA figures yields a pair of intersecting curves almost identical in slope with those for respiration, and with a break on the eighth day. Thus a number of lines of evidence suggest that the period around the eighth day of development is one of transition in the chemical growth of the chick embryo.

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